23. Molecules as diverse as cholesterol, sex hormones, and cortisone all contain common structural elements. Write the structural formula that represents the common structural elements.

27. Carbon usually forms four covalent bonds, nitrogen usually forms three bonds, oxygen usually forms two bonds, and hydrogen can only form one bond. Use this information to write structural formulas for each of these compounds.

a. A compound that contains one carbon atom, one nitrogen atom, and as many hydrogen atoms as needed.
b. A compound that contains one carbon atom, one oxygen atom, and as many hydrogen atoms as needed.

29. Styrene, C H CH==CH is another compound that contains a ring similar to
6 5 2
Benzene, C H
6 6.
One hydrogen has been replaced by the side chain ----CH==CH
2.
Write a set of structural formulas for styrene to show that this molecule, like benzene, also has resonance structures.

35. Vitamin E is often sold as a racemic mixture of D- and L-isomers.

a. Which is the more physiologically active isomer?
b. How does the cost of the racemic mixture compare with the price of the pure, physiologically active isomer?